As one form of the optical communication network, a passive optical network (PON) of a configuration, for example, as illustrated in FIG. 1 is proposed and is applied to an optical access network or the like. In the PON illustrated in FIG. 1, an OLT (Optical Line Terminal) 101 which is disposed on the side of a communication server is connected with a plurality of ONUs (Optical Network Units) 104 which are disposed on the sides of users via an optical fiber 102 and an optical splitter 103. For example, 32 to 64 ONUs 104 are disposed. Signal light is sent and received bi-directionally between the OLT 101 and each ONU 104. For example, in a network which is called a G-PON (Gigabit-Passive Optical Network) or a GE-PON (Gigabit Ethernet-Passive Optical Network), signal light is sent and received at a communication speed of 1 gigabit/sec (Gbps). In a general type PON as mentioned above, a communication path between the OLT 101 and each ONU 104 is configured by using only passive components. Thus, in general, a redundancy optical circuit is not adopted and it may be basically unnecessary to perform supervisory control on a device disposed on the communication path. For example, a distance between the OLT 101 and each ONU 104 ranges from about 10 km to about 20 km.
Incidentally, in a PON of the above mentioned type, it is expected that needs will emerge for increasing a transmission distance and the number of users (the number of branch signals which are obtained by using an optical splitter) with increasing the communication speed of the optical access network. In general, in order to promote elongation of the communication distance in the G-PON or the GE-PON, it is proposed to cope with elongation of the communication distance by converting an optical signal to an electric signal on an intermediate part of the communication path, performing optical 2R regeneration including a reamplification function and a reshaping function or optical 3R regeneration including the reamplification function, the reshaping function and a retiming function on the signal and again sending the regenerated signal light onto the communication path. However, as a configuration to be applied to a network handling high-speed signal light which is higher than 10 Gbps such as a 10GE-PON, an XG-PON or the like, the configuration that performs the optical 2R or 3R regeneration on the signal on an intermediate part of the communication path may be costly. In particular, up-signal light which is directed from each ONU to the OLT in the optical access network is sent in the form of an optical burst signal. Thus, it may be desired to install a high-speed optical burst signal receiver in order to perform the optical 2R or 3R regeneration on the signal on an intermediate part of the communication path and hence the cost of the configuration may be relatively increased.
Though different from the PON, a system in which an optical repeater (optical repeating installation) which is configured by using an optical amplifier or the like is disposed on a communication path and the amplitude of signal light is amplified as it is without being converted to an electric signal and is relay-transmitted, instead of performing the optical 2R or 3R regeneration on the signal on an intermediate part of the communication path is proposed for use in an optical communication network which is adopted in a main line system or the like. In the above mentioned system, in order to smoothly perform duties such as sending of a notification when a failure has occurred, periodical maintenance and others, supervisory control of the optical repeater using an active component may be desired.
In general, in an existing optical communication network, supervisory control of the optical repeater or the like is performed by using a circuit which is different from a circuit used for signal light communication or by using a wavelength which is different from the wavelength of the signal light so as to transmit supervisory control signal light on the network. In a supervisory control system as mentioned above, the transmission capacity of the signal light is limited owing to transmission of the supervisory control signal light and hence the above mentioned system may be unsuited to construct an optical communication network which aims at cost reduction such as an optical access network or the like.
As an existing technique for realizing supervisory control of an optical repeater or the like at a low cost, a system of transmitting supervisory control information using a circuit and a wavelength which are the same as those used for transmission of a signal light, for example, by carrying the supervisory control information on a tone signal and superimposing the tone signal on the signal light is proposed. In addition, a technique for transmitting supervisory control information by superimposing a tone signal on ASE (Amplified Spontaneous Emission) light which is generated using an optical amplifier such as an erbium doped fiber amplifier (EDFA) or the like is proposed.
However, in an existing system of transmitting the supervisory control information by superimposing the tone signal on the signal light as described above, if the signal light is not present, transmission of the supervisory control information may be impossible and hence the above mentioned system may not be applied to a network on which an intermittent optical burst signal is transmitted as in the case of transmission of the up-signal light from each ONU to the OLT on the above mentioned optical access network.